Rubber obtained by hydrogenating acrylonitrile-butadiene copolymer rubber (NBR) is well known as an excellent rubber having a good balance among oil resistance, heat resistance, weather resistance, low-temperature resistance and strength. Since hydrogenated NBR has such excellent characteristics, it is used for purposes requiring oil resistance, heat resistance and strength, for example, it is used in toothed transmission belts (timing belts) for automobiles.
However, also in such applications, from the viewpoint of long-term service, a high strength at ordinary temperature and high temperatures is required, and in order to insure efficient power transmission it has been desirable to improve the insufficient low-extension stress (e.g. 50% tension stress) at high temperatures which is a defect of hydrogenated NBR.
On the other hand, it is known that the strength of NBR can be improved by further copolymerizing a carboxyl-group-containing monomer. However, although excellent in strength at ordinary temperature and high temperatures, a hydride of the NBR obtained by copolymerizing a carboxyl-group-containing monomer is not much different from hydrogenated NBR in low-extension stress at high temperature, namely, it is not improved in low-extension stress.